Amongst the many beauties of Orkney are the beaches which fringe the bays and ayres of the inner coast. These are dynamic forms, changing shape with the seasons and gradually retreating landward as sea level rises. The most dramatic changes occur during storms, when sand and gravel ridges migrate or are breached by sea. The lagoons or oyces may switch between fresh and salt water. Over time, the lagoons fill with sediment and vegetation to leave reed-covered bogs. These features are typical of submerging coastlines.